Control system for a vibrating structure gyroscope

ABSTRACT

A vibrating structure gyroscope having a vibrating structure ( 3 ) primary drive means ( 4 ) and secondary drive means ( 7 ) and primary pick-off means ( 2 ) an secondary pick-off means ( 6 ) is provided with a control system. The control system includes a primary closed control loop ( 1 ) for controllably varying the drive signal applied to the primary drive means ( 4 ), a secondary closed control loop ( 5 ) for controllably varying the drive signal applied to the secondary drive means ( 7 ) in order to maintain a null value of the secondary pick-off means ( 6 ) and means ( 30 ) for actively adjusting the scalefactor in the primary and secondary closed control loops ( 1, 5 ). The means ( 30 ) includes means ( 34 ) for dividing a rate response signal from the loop ( 5 ) by a signal indicative of the amplitude of the primary mode vibration. The output form the means ( 34 ) is filtered to provide an applied rate output. A variable scalefactor loop ( 3 ) uses the output from the means ( 34 ) actively to adjust a reference voltage set level of loop ( 1 ) to adjust the in loop scalefactor of the system.

This application is the US national phase of international applicationPCT/GB01/01381 filed Mar. 28 2001, which designated the US.

FIELD OF THE INVENTION

This invention relates to a control system for a vibrating structuregyroscope particularly, but not exclusively, suitable for use with asilicon micro-machined vibrating structure gyroscope.

Vibrating structure gyroscopes are known using a variety of differentmechanical vibratory structures. These include beans, tuning forks,cylinders, hemispherical shells and rings made from ceramic, metal orsilicon. A common feature in these known systems is that they arerequired to maintain a resonance carrier mode oscillation at a naturalfrequency determined by the mechanical vibratory structure. Thisprovides the linear momentum which produces Coriolis force when thegyroscope is rotated around the appropriate axis.

Advances in micro-machining technology have made it possible to producevibrating structure gyroscopes from silicon in high volumes and at lowcost. Such gyroscopes are being developed for automotive applicationssuch as vehicle dynamic control systems and for car navigation. Theperformance characteristics of these micro-machined gyroscopes aretailored to meet automotive requirements with the maximum specified raterange typically being plus or minus 100° per second.

Such micro-machined vibrating structure gyroscopes are inherently ruggedand of low cost which makes them attractive for use in other moredemanding applications such as for aircraft navigation or for guidedmunitions. These latter applications typically require the gyroscope tooperate over a significantly wider range of rotation rates. Whilst it ispossible to extend the rate range capability of gyroscopes developed forautomotive applications this will typically result in degradation ofother key performance parameters such as noise and bias.

Conventional vibrating structure gyroscopes having a planar ringvibrating structure made of metal or silicon or having a cylindricalvibrating structure give good overall performance. Planar ring vibratingstructures are typically driven in Cos 2 θ vibration modes as shownschematically in FIGS. 1a and 1 b of the accompanying drawings. Onemode, having radial anti-nodes aligned along axes P, as shown in FIG.1a, is excited as the primary mode. When the gyroscope is rotated aroundthe axis normal to the plane of the ring Coriolis forces F_(c) aredeveloped which couple energy into the secondary mode, whose radialanti-nodes are aligned along axes S, as shown in FIG. 1b. The magnitudeof the force is given by:

F _(c)=2 mvΩ_(app)  (1)

where m is the modal mass, v is the effective velocity and Ω_(app) isthe applied rotation rate. The primary mode vibration amplitudetypically is maintained at a fixed level. This also maintains thevelocity, v, at a fixed level and hence ensures that the developedCoriolis forces are directly proportional to the rotation rate, Ω_(app).The amplitude of secondary mode motion induced by these Coriolis forcesmay conventionally be enhanced by accurately matching the resonantfrequencies of the primary and secondary modes. The motion is thenamplified by the Q (measure of the relation between stored energy andthe rate of dissipation of energy) of the secondary mode giving enhancedvibrating structure gyroscope sensitivity. When operating in this openloop mode the sensitivity (scalefactor) of the gyroscope will bedependent on the Q of the secondary mode which may vary significantlyover the operating temperature range. This dependence may be eliminatedby operating the gyroscope in a force feedback (closed loop) mode. Inthis mode the induced secondary mode motion is actively nulled with theapplied force being directly proportional to the rotation rate.

A typical conventional closed loop control system for a vibratingstructure gyroscope is shown schematically in FIG. 2 of the accompanyingdrawings. This conventional control system basically consists of twoindependent loops namely a primary loop 1 between a primary pick-offmeans 2 which acts as a motion detector output from the vibrating planarring structure 3 and a primary drive means 4 which acts as a forcinginput creating vibration in the structure 3. A secondary loop 5 isprovided between a secondary pick-off means 6 and a secondary drivemeans 7.

The output signal 8 from the primary pick-off means 2 is amplified by anamplifier 9 and demodulated by demodulators 10 and 11. The demodulatedsignal from the demodulator 10 is passed first to a phase locked loop 12which compares the relative phases of the primary pick-off and primarydrive signals at the means 2 and 4 and adjusts the frequency of avoltage control oscillator 13 to maintain a 90° phase shift between theapplied drive at means 4 and the resonator motion of the structure 3.This maintains the motion of the structure 3 at the resonance maximum.The demodulated output from the demodulator 11 is supplied to anautomatic gain control loop 14 which compares the level of the outputsignal from the primary pick-off means 2 in the automatic gain controlloop 14 to a fixed reference level V_(o). This signal V_(o) is appliedat 15 to a voltage adder 16 and the output voltages therefrom suppliedto the automatic gain control loop 14. The output voltage, from theautomatic gain control loop 14 is remodulated at remodulator 17 at thefrequency supplied by the voltage controlled oscillator 13 and then fedvia an amplifier 18 to the primary drive means 4. The primary drivevoltage level is adjusted in order to maintain a fixed signal level, andhence amplitude of motion, at the primary pick-off means 2.

The secondary loop 5 is such that the signal received from the secondarypick-off means 6 is amplified by amplifier 19 and demodulated bydemodulators 20 and 21 to separate real and quadrature components of therate induced motion. The real component is that which is in-phase withthe primary mode motion. The quadrature component is an error term whicharises due to the mode frequencies not being precisely matched. Thedemodulated baseband signal received from the demodulator 20 is filteredby a quadrature loop filter 22 and the demodulated baseband signalreceived from the demodulator 21 is filtered by a real loop filter 23 toachieve the required system performance in respect of bandwidth andnoise. The signal received from the loop filter 22 is remodulated atremodulator 24 and passed to a voltage adder 25 where it is summed withthe signal received from the loop filter 23 after remodulation byremodulator 26. The summed output signal from the voltage adder 25 isapplied to the secondary drive means 7 via an amplifier 27 to maintain anull at the secondary pick-off unit 6. The real baseband signal SD(real), which is the output signal from the real loop filter 23 is takenoff before remodulation at the remodulator 26, scaled and filtered atoutput filter 28 to produce the rate output signal 29 from the system.The real baseband signal SD (real) is directly proportional to the realsecondary drive applied to the vibrating structure 3.

For this mode of operation the rate output Ω_(out,) is given by:$\begin{matrix}{\Omega_{out} = \frac{k\quad {{SD}({real})}g_{ppo}g_{sd}}{V_{o}w}} & (2)\end{matrix}$

where V_(o) is the fixed primary mode amplitude reference voltage setlevel, w is the primary mode resonance frequency, k is a constantincluding the modal mass and modal coupling coefficient, g_(ppo) is theprimary mode pick-off gain and g_(sd) is the secondary mode driver gain.

For a gyroscope operating in this conventional closed loop mode, theminimum detectable rotation rate that can be resolved is determined bythe sensitivity of the secondary mode pick-off means 6. This isdetermined by the electronic noise of the secondary pick-off amplifier19. For a fixed pick-off gain, the only way to enhance the resolution ofthe vibrating structure gyroscope is to increase the secondary modemotion generated by a given applied rate, that is to increase the inloop scalefactor. This may be achieved by increasing the drive level ofthe primary mode to give a larger amplitude of motion, that is toincrease the primary mode amplitude set level V_(o). In practice therewill be a limit to the maximum displacement for any given vibratingstructure gyroscope design which limit may be set by a number of factorswhich include the available drive force, the fracture limit of thevibrating structure and non-linearities in the pick-off and drive meanswhich conventionally are inductive, capacitive or piezo electrictransducers.

The maximum applied rate that the vibrating structure gyroscope canmeasure is limited by the ability of the secondary drive means 7 tomaintain a null in the secondary mode motion. The secondary drive means7 applies a force to null the induced Coriolis force. For rotation ratesabove a certain level the magnitude of the Coriolis force is such thatsecondary drive can no longer apply sufficient force to null the motioncausing the rate output to saturate. For typical conventional vibratingstructure gyroscopes this saturation level corresponds to appliedrotation rates of less than a few hundred degrees per second. For agiven secondary drive means gain it is possible to increase the maximummeasurable applied rate. This is achieved by reducing the primary modeamplitude which reduces the magnitude of the induced Coriolis force fora given rotation rate. However this solution will degrade the signal tonoise performance of the vibrating structure gyroscope thus reducing theresolution undesirably.

There is thus a need for an improved control system for a vibratingstructure gyroscope which is capable of measuring greater rotation rateswhilst maintaining high resolution.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided acontrol system for a vibrating structure gyroscope having a vibratingstructure, primary drive means and secondary drive means for putting andmaintaining the vibrating structure in primary mode vibratory resonance,and primary pick-off means and secondary pick-off means for detectingvibration of the vibrating structure, which system includes a primaryclosed control loop for controllably varying the drive signal applied tothe primary drive means, a secondary closed control loop forcontrollably varying the drive signal applied to the secondary drivemeans in order to maintain a null value at the secondary pick-off means,and means for actively adjusting the scalefactor in the primary andsecondary closed control loops, which scalefactor active adjustmentmeans includes means for dividing a rate response signal from thesecondary control loop by a signal indicative of the amplitude of theprimary mode vibration, means for filtering an output signal from thedividing means to provide an output indicative of the applied rate, anda variable scalefactor loop for receiving the output signal from thedividing means and using it actively to adjust a reference voltage setlevel of the primary closed control loop and thereby dynamically adjustthe in loop scalefactor of the control system.

Preferably the scalefactor active adjustment means includes means forreducing the primary mode vibration amplitude for applied rotation ratesabove a selected absolute threshold rate value.

Conveniently the absolute threshold rate value selected is set at avalue less than the rate output limit of the secondary closed controlloop.

Advantageously the primary closed control loop includes means fordemodulating the signal received from the primary pick-off means, aphase locked loop for comparing the relative phases of the primarypick-off and primary drive signals, a voltage controlled oscillator thefrequency of which is adjusted by the phase locked loop to maintain a90° phase shift between the signals supplied to the primary drive meansand the motion of the vibrating structure, an automatic gain controlloop for comparing the demodulated signal received from the primarypick-off means to a fixed reference voltage level, and a modulator forremodulating the output signal received from the automatic gain controlloop at the frequency supplied by the voltage controlled oscillation toprovide the controllably varied drive signal supplied to the primarydrive means.

Preferably the secondary closed control loop includes means fordemodulating and splitting the signal received from the secondarypick-off means into the real component and the quadrature component of arate induced motion of the vibrating structure, loop filtering means forseparately filtering the real and quadrature components, and means forremodulating and summing the filtered signal components for applicationto the secondary drive means.

Conveniently the variable scalefactor loop is connected between thedemodulated output from the real component loop filtering means of thesecondary closed control loop and the demodulated signal from theprimary pick-off means.

Advantageously the variable scalefactor loop includes means for dividingthe modulus of the input signal applied to the variable scalefactor loopinto a fixed voltage reference level with the output limited to valuesless than or equal to one, and means for filtering the output and forusing the filtered output for scaling the zero rate voltage value of thefixed reference voltage level of the automatic gain control loop of theprimary closed control loop.

Preferably the control system includes means located between thesecondary closed control loop and the variable scalefactor loop todivide the demodulated output from the real component loop filteringmeans of the secondary closed control loop by the demodulated signalfrom the primary closed control loop to provide an output signalproportional to the applied rate, which output signal forms the input tothe variable scalefactor loop.

Alternatively the control system includes means located between thesecondary closed control loop and the variable scalefactor loop todivide the demodulated output from the real component loop filteringmeans of the secondary closed control loop by the reference voltagelevel forming the output from the variable scalefactor loop, to providean output signal proportional to the applied rate, which output signalforms the input to the variable scalefactor loop.

Conveniently the control system includes means for taking off part ofthe output signal forming the input to the variable scalefactor loop,scaling it and filtering it to provide an output signal indicative ofthe rate applied to the gyroscope.

Advantageously the control system is used with a vibrating structuregyroscope having a vibrating structure made from silicon.

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings, in which:

DESCRIPTION OF THE DRAWING

FIG. 1a shows diagrammatically for a vibrating structure gyroscope notaccording to the present invention a Cos2 θ vibration mode which isexcited as the primary mode,

FIG. 1b shows schematically a Sin2 θ vibration mode excited when thevibrating structure gyroscope, not according to the present invention,is rotated around an axis normal to the plane of the vibrating structureso that Coriolis forces are developed which couple energy into thesecondary mode,

FIG. 2 is a schematic block diagram of a conventional control system notaccording to the present invention for a vibrating structure gyroscope,

FIG. 3 is a generalised block diagram of a control system according to afirst embodiment of the present invention,

FIG. 4 is a generalised block diagram of a control system according to asecond embodiment of the present invention, and

FIG. 5 is a schematic block diagram showing the functionality of avariable scalefactor loop utilised in a control system of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A control system according to a first embodiment of the presentinvention for a vibrating structure gyroscope is shown in FIG. 3 of theaccompanying drawings. In this embodiment of the invention the primaryloop 1 and secondary loop 2 are very similar to the primary loop 1 andsecondary loop 2 of the conventional control system shown in FIG. 2 andlike components common to both FIGS. 2 and 3 will be given likereference numerals and not commented on further in any detail.Additionally although the control system of the present invention isshown and described as applied to a vibrating structure 3 made fromsilicon it can also be applied to the vibrating structure 3 made frommetal or piezo ceramic material.

Adjusting the scalefactor as previously described has the problem, thatonce set, the scalefactor cannot subsequently be varied. In order toextend the rate range of the vibrating structure gyroscope withoutcompromising the lower rate performance it is necessary actively toadjust the in-loop scalefactor at higher applied rates. A control systemaccording to the present invention includes means 30 in the primary andsecondary closed control loops 1, 5 to enable the gyroscope to measurehigh applied rotation rates whilst maintaining optimum resolution at lowrotation rates. To this end the scalefactor active adjustment means 30includes means for reducing the primary mode vibration amplitude forapplied rotation rates above a selected absolute threshold rate value.The applied rates are positive or negative. The absolute threshold rateΩ_(Th) value is conveniently set at a value slightly below the normaloutput limit of the secondary drive means 7.

For applied rates below the threshold value the primary mode amplitudeis maintained at a constant value. The secondary mode motion induced bythe Coriolis forces will be directly proportional to the applied rateand is nulled in a conventional manner using the secondary drive means7. As the applied rate exceeds the threshold value the primary modeamplitude is reduced therefore decreasing the secondary loopscalefactor. In this rate range the secondary drive is maintained arounda constant, maximum, level and the primary mode amplitude variesinversely proportional to the applied rate. This is achieved byadjusting the primary mode amplitude reference voltage set level V_(AGC)in the following manner:

V _(AGC)∝1/Ω_(APP)  (3)

To obtain a linear rate output over the entire operating range it isnecessary to use the secondary drive level normalised using the primarymode amplitude. The rate output Ω_(out) is given by: $\begin{matrix}{\Omega_{out} = {k\quad \frac{{SD}({real})g_{PPO}g_{SD}}{V_{PPD}w}}} & (4)\end{matrix}$

where V_(PPD) is the primary pick-off signal indicative of the primarymode amplitude.

To carry this into effect and provide an extended rate range capability,as shown in FIG. 3 the scalefactor adjustment means 30 are providedconnected between the secondary closed loop 5 and the primary closedcontrol loop 1. The means 30 includes means 34 located between thesecondary closed control loop 5 and the variable scalefactor loop 31 fordividing the demodulated output 32 from the real component loopfiltering means 23 of the secondary closed control loop 5 by thedemodulated signal component 33 from the primary closed control loop 1to provide an output signal 35 proportional to the applied rate. Theoutput 35 forms the input to a variable scalefactor loop 31 (formingpart of the means 30, and is used to scale the voltage reference level15 supplied to the automatic gain control loop 14 of the primarypick-off signal 33. This output signal 35 is also scaled and filtered bymeans 36 to provide a final output signal Ω_(OUT) which is indicative ofthe rate applied to the gyroscope.

Alternatively as shown in FIG. 4 of the accompanying drawings, the means30 for actively adjusting the scalefactor may be implemented using theinput voltage 15 instead of the demodulated other signal component 33 ofthe primary closed control loop 1 without changing the basicfunctionality of the control system. Thus as shown in FIG. 4 the means34 which is located between the secondary control closed loop 5 and thevariable scalefactor loop 31 to divide the demodulated output 32 fromthe real component loop filtering means 23 of the secondary closedcontrol loop 5 by V_(AGC) the reference voltage level 15 forming theoutput from the variable scalefactor loop 31 provides an output directcurrent signal 35 proportional to the applied rate, which output signalforms the input to the variable scalefactor loop 31. The signal 35 isalso scaled and filtered as at 36 to provide the final output signal 37(Ω_(out)) which is indicative of the rate applied to the gyroscope.

The functionality of the variable scalefactor loop 31 is shownschematically in FIG. 5. The modulus of the input signal 35 (Ωout)applied to the variable scalefactor loop 31 is divided at 38 into afixed voltage reference level indicative of Ω_(TH) with the output valueX limited to less than or equal to 1 at 39. This reference level setsthe threshold rate value Ω_(Th) and effectively limits the maximum realdrive amplitude which may be applied to the secondary drive means 7. Theoutput X is then used at 40 to scale V_(θ), the zero rate value of theautomatic gain control reference voltage level V_(AGC) (input 15). Theoutput 41 from 40 is filtered in a loop filter 42 to provide therequired dynamic response prior to application to the automatic gaincontrol loop 14.

The variable scalefactor loop filtering has a critical role indetermining the dynamic performance of the control system. As thebandwidth of the variable scalefactor loop 31 typically is low comparedto that of the secondary (real) control loop 5, the primary amplitudewill respond relatively slowly to rapid changes in the applied rate inthe region above the threshold value Ω_(Th) this means that the null atthe secondary pick-off means 6 is predominantly maintained by adjustingthe secondary drive means 7 under the control of the secondary controlloop 5. For rapid increases in Ω_(APP) this will cause the instantaneoussecondary drive levels to exceed the steady state condition for theequivalent applied rates. This will require that Ω_(Th) be set toprovide sufficient overdrive range to prevent the secondary drive“overshooting” and exceeding the output limit under these transientconditions. This is detrimental to the performance of this system as itrestricts the usable dynamic range of the secondary control loop 5 undernormal conditions, that is where there are no rapid changes in Ω_(APP).Extending the bandwidth of the variable scalefactor loop 31 will limitthe extent of the secondary drive overshoot by enabling the automaticgain control loop 14 to respond more rapidly. Extending the band widthwill, however, degrade the noise performance of the system.

The detailed loop filter design of the control system according to thepresent invention is dependant upon the precise performancerequirements. The usable rate range of the vibrating structure gyroscopeto which it is applied, and which preferably has a vibrating structuremade from silicon may be extended from the typical 100° per second to inexcess of 10,000° per second. This is achieved without degradation inperformance at the lower applied rates. There will, however, be somedegradation of the noise performance as the rate is increased above thethreshold level due to the reduced amplitude of primary motion.

What is claimed is:
 1. A control system for vibrating structuregyroscope having a vibrating structure, primary drive means andsecondary drive means for putting and maintaining the vibratingstructure in primary mode vibratory resonance, and primary pick-offmeans and secondary pick-off means for detecting vibration of thevibrating structure, which system includes a primary closed control loopfor controllably varying the drive signal applied to the primary drivemeans, a secondary closed control loop for controllably varying thedrive signal applied to the secondary drive means in order to maintain anull value at the secondary pick-off means, and means for activelyadjusting the scalefactor in the primary and secondary closed controlloops which scalefactor active adjustment means includes means fordividing a rate response signal from the secondary control loop by asignal indicative of the amplitude of the primary mode vibration, meansfor filtering an output signal from the dividing means to provide anoutput indicative of the applied rate, and a variable scalefactor loopfor receiving the output signal from the dividing means and using itactively to adjust a reference voltage set level of the primary closedcontrol loop and thereby dynamically adjust the in loop scalefactor ofthe control system.
 2. A control systems according to claim 1, whereinthe scalefactor active adjustment means includes means for reducing theprimary mode vibration amplitude for applied rotation rates above aselected absolute threshold rate value.
 3. A control system according toclaim 2, wherein the absolute threshold rate value selected is set at avalue less than the rate output limit of the secondary closed controlloop.
 4. A control system according to claim 1, wherein the primaryclosed control loop includes means for demodulating the signal receivedfrom the primary pick-off means, a phase locked loop for comparing therelative phases of the primary pick-off and primary drive signals, avoltage controlled oscillator the frequency of which is adjusted by thephase locked loop to maintain a 90° phase shift between the signalapplied to the primary drive means and the motion of the vibratingstructure, an automatic gain control loop for comparing the demodulatedsignal received from the primary pick-off means to a fixed referencevoltage level, and a modulator for remodulating the output signalreceived from the automatic gain control loop at the frequency suppliedby the voltage controlled oscillator to provide the controllably varieddrive signals applied to the primary drive means.
 5. A control systemaccording to claim 1, wherein the secondary closed control loop includesmeans for demodulating and splitting the signal received from thesecondary pick-off means into the real component and of the quadraturecomponent of rate induced motion of the vibrating structure, loopfiltering means for separately filtering the real and quadraturecomponents, and means for remodulating and summing the filtered signalcomponents for application to the secondary drive means.
 6. A controlsystem according to claim 4, wherein the variable scalefactor loop isconnected between a demodulated output from the real component loopfiltering means of the secondary closed control loop and the demodulatedsignal from the primary pick-off means.
 7. A control system according toclaim 6, wherein the variable scalefactor-loop includes means fordividing the modulus of the input signal applied to the variablescalefactor loop into a fixed voltage reference level with the outputlimited to values less than or equal to one, and means for filtering theoutput and for using the output for scaling the zero rate voltage valueof the fixed reference voltage level to the automatic gain control loopof the primary closed control loop.
 8. A control system according toclaim 7, including means located between the secondary closed controlloop and the variable scalefactor loop to divide the demodulated outputfrom the real component loop filtering means of the secondary closedcontrol loop by the demodulated signal from the primary closed controlloop to provide an output signal proportional to the applied rate, whichoutput signal forms the input to the variable scalefactor loop.
 9. Acontrol system according to claim 7, including means located between thesecondary closed control loop and the variable scalefactor loop todivide the demodulated output from the real component loop filteringmeans of the secondary closed control loop by the reference voltagelevel forming the output from the variable scalefactor loop to providean output signal proportional to the applied rate, which output signalforms the input to the variable scalefactor loop.
 10. A control systemaccording to claim 8, including means for taking off part of the outputsignal forming the input to the variable scalefactor loop, scaling itand filtering it to provide an output signal indicative of the rateapplied to the gyroscope.
 11. A control system according to claim 1,when used with a vibrating structure gyroscope having a vibratingstructure made from silicon.